add `%%px --local` for local execution

So that a cell can be run on engines *as well as* the local namespace.

Prompted by SO Question [14184621](http://stackoverflow.com/questions/14184621).
%%px --local runs local last

ensures concurrent execution of all n+1 executions
finish up return value for %%px --local
add %%px --local to Parallel Magics notebook
Prefilter shouldn't touch execution_count
Merge pull request #2783 from takluyver/prefilter-dont-touch-execution-count

Prefilter shouldn't touch execution_count
Merge pull request #2753 from minrk/pxlocal

add `%%px --local` for local execution

local execution is concurrent with remote.